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If you want to change all font size in a form, you just have to click preferences -> form styles -> font size. Change this value to any you want.
Selecting form styles. Here you can change the font size to any value you want.
But, if you mean the font size in a email notification, you have to select the text you want and change its size in the button as I show you here:
Hope it can help. If you have any problem or this doesn't help you, please post it.
I understand that you want to increase the font size of the submissions while printing. If so, unfortunately, it is not controlled by JotForm but the application that is being used i.e., the browser. Please refer to the website below that might help you in increasing the font size while printing the submissions.
If you are referring to something different, please let us know. We will be happy to assist.